1. Field of the Invention
This invention relates to plastic molding having satin finish type metallic luster.
2. Description of the Prior Art
Satin finishing has been applied to the surface of metallic article so as to provide good appearance thereto. The treatment is carried out using abrasive grains such as iron powder, glass powder, silicon oxide, carborundum, alumina and the like.
The treatment can be easily carried out and no technical difficulties have been encountered. However, in the satin finishing of articles other than metallic articles, there have been many problems to be solved.
The typical one is the satin finishing of plastic articles. Recently, plastic molding has been plated so as to coat the surface thereof with a metal so that the molding may be provided with metallic appearance. The satin finish is applied to the metal coating and thus a plastic molding having satin finish type metallic luster is obtained.
For the satin finishing of plastic molding, two methods have been conventionally employed. One of them is a method in which the surface of plastic molding is roughened by a mixture of chromic acid, sulfuric acid, and phosphonic acid, is subjected to pre-treatment by stannous chloride SnCl.sub. 2) and palladium chloride, is provided with a conductive layer (for instance by applying chemical plating using metallic salt solution with a reductant dissolved therein), and is provided with a metal film of proper thickness (for example, several to ten microns; by applying electric plating using acid copper salt plating bath) to thereby produce a satin finish type metallic luster thereon. In order to provide desired appearance, plating using nickel, chromium, gold, rhodium and the like may be applied. This method is of practical value in the satin finishing, but there are many problems to be solved.
The metal coating should be set to a thickness sufficient to be durable to the satin finish, i.e., generally more than 20.mu.. Thus, the formation of the metal film requires a long period, e.g., 0.5 to 1 hours, which is not preferred from the economic point of view. Furthermore, there occurs a problem that there are produced uneven portions because of difference of current density during plating, e.g. thin portion is about 20.mu. thick and thick portion is about 200.mu. thick. Such unevenness of the metal film reduces accuracy and appearance, thereby seriously deteriorating the product value. Moreover, since the satin finish is applied after formation of metal film, the molding is deformed (since metal is of high malleability and the abrasive grain is sprayed thereon). In addition, a part of the metal film is often peeled off, and thus appearance and accuracy of the finished product is deteriorated. For these reasons, the above method is not applicable to a plastic molding for which good appearance and accuracy is required.
According to the other method, the satin finishing is applied directly to a plastic article to form a conductive coating thereon and then a metal film is formed on the coating by electric plating. This method can avoid the foregoing defects such as the limited thickness and the deformation of metal film, and the like. However, since the treatment is applied directly on the molding, there occurs other problems and it is not possible to produce excellent finished product. That is, since plastic molding has defects which are not visible such as weed mark, sink, stress during molding and the like, a finished product suffers from stain of conductive film, difference of luster, coarse surface and the like during the formation of conductive film. These defects cannot be removed by plating to the thickness of 10 to 15.mu.. Thus, the final product has stain, difference of luster and is coarse, an article having good appearance cannot be produced.
On the other hand, when plating is carried out to remove the defects to the thickness more than 20.mu., the satin finish is made even or flat by the plating and a good satin finish can not be obtained.